Conventionally, there were known lighting apparatuses for lighting a high-brightness discharge lamp (hereinafter, referred to as an HID lamp) such as a mercury lamp and a metal halide lamp. Such lighting apparatuses employ a high-voltage pulse generator (also referred to as a discharge lamp igniter) for instantaneously starting up or restarting up the HID lamp, in addition to an inverter for supplying power to the HID lamp.
As shown in FIG. 13, a high-voltage pulse generator 100 includes an IGN connector A having in put terminals IN1 to IN3 electrically connected to output terminals of an inverter and a socket B into which a cap E of an HID lamp DL is fitted. The high-voltage pulse generator 100 employs a pulse transformer (not shown) for converting a low-voltage pulse into a high-voltage pulse.
As shown in FIG. 14, such a high-voltage pulse generator 100 includes a pulse transformer PT in which a primary winding N1 and a secondary winding N2 are wound on a cylinder-shaped ferrite core 200, terminals 201 and 201 electrically connected to both ends of the first primary winding N1, respectively, a high-voltage part terminal 202 and a low-voltage part terminal 203 electrically connected to both ends of the secondary winding N2, respectively, an insert molded member 204 into which the pulse transformer PT and the terminals 201 to 203 are inserted with the terminals 201 to 203 exposed, and a case 206 into which the insert molded member 204 is inserted, on which electronic components 205 constituting the high-voltage pulse generator 100 along with the pulse transformer PT are mounted, and which includes a socket B having an inner electrode OUT1 and an outer electrode OUT2 connected to an inner electrode and an outer electrode of the cap E of the HID lamp DL (Patent Document 1).
A circuit configuration of the high-voltage pulse generator 100 is now described with reference to FIG. 15. As shown in FIG. 15, the high-voltage pulse generator 100 includes a high-voltage part input terminal IN1 and low-voltage part input terminals IN2 and IN3, which constitute the IGN connector A, electrically connected to the output terminals of the inverter, a pulse generating capacitor C connected between the input terminals IN1 and IN3, a surge absorber ZNR such as a two-way diode and a varistor connected between the input terminals IN1 and IN2, a resistor R connected in parallel to the capacitor C so as to discharge electric charges remaining in the capacitor C and the surge absorber ZNR, a pulse transformer PT having a primary winding N1 connected in parallel to the capacitor C and a secondary winding N2 disposed between the input terminal IN1 and the inner electrode OUT1 of the socket B, and a high-voltage pulse generating discharge switch (discharge gap) SG for switching a discharge path from the capacitor C to the primary winding N1. The input terminal IN2 is directly connected to the outer electrode OUT2 of the socket B. Accordingly, in the circuit of the high-voltage pulse generator 100, a portion denoted by reference numeral HV in FIG. 15 serves as a high-voltage part circuit and a portion denoted by a reference numeral LV serves as a low-voltage part circuit. Specifically, a loop circuit, which is denoted by a reference numeral P, including the capacitor C, the primary winding N1, and the discharge switch SG forms a pulse generator through which large current, that is, pulses flows in the low-voltage part circuit LV.
Next, an operation of the high-voltage pulse generator 100 is described. When an output of the inverter is input to the input terminals IN1 to IN3 of the high-voltage pulse generator 100, the capacitor C is charged by a potential difference between the input terminals IN1 and IN3. When the voltage of the capacitor C becomes greater than a predetermined value, the discharge switch SG is turned on and a pulse is applied to the primary winding N1. In this way, when the pulse is applied to the primary winding N1 of the pulse transformer PT, the pulse transformer PT outputs a high-voltage pulse out of the secondary winding N2. Accordingly, the high-voltage pulse is supplied from the inner electrode OUT1 to the HID lamp DL and thus the HID lamp DL is ignited or re-ignited.
The high-voltage pulse generator 100 described above is used to instantaneously ignite and re-ignite the HID lamp in a lighting apparatus for lighting a HID lamp such as a mercury lamp and a metal halide lamp. A vehicle headlight apparatus 300 shown in FIG. 16 is an example of such a lighting apparatus.
As shown in FIG. 16, the vehicle headlight apparatus 300 includes a box-shaped lamp housing 310, a reflecting plate 320 for reflecting light emitted from an HID lamp DL, a high-voltage pulse generator 100 into which the HID lamp DL is fitted, and an inverter 330 electrically connected to the high-voltage pulse generator 100 so as to convert a DC voltage supplied from a 12V battery (not shown) of a vehicle into an AC voltage for driving the HID lamp.
The lamp housing 310 is formed in a box shape of which the front surface (left surface in FIG. 16) is opened and is mounted with a front lens 311 to cover the opened front surface. In a rear wall of the lamp housing 310, a circular lamp inserting hole 310a for inserting the HID lamp DL into the lamp housing 310 is formed in a portion corresponding to the back of the HID lamp DL disposed in the lamp housing 310. A recessed portion 340a for receiving a part of the high-voltage generator 100 is formed in the circumferential edge of the lamp inserting hole 310a, and a lamp interchanging maintenance cap 340 for closing the lamp inserting hole 310a is attached to the recessed portion 340a to be detachable from the back side. In addition, a power line inserting hole 310b for introducing a power line such as a harness H having one end connected to the inverter 330 into the lamp housing 310 is formed in the lower wall of the lamp housing 310. A screw inserting hole (not shown) through which an optical axis adjusting screw 350 is inserted to pass through the rear wall is formed in the vicinity of the lamp inserting hole 310a formed in the rear wall of the lamp housing 310.
The reflecting plate 320 is vertically rotatably received in the lamp housing 310 in the state that the reflecting surface is directed to the front side. The reflecting plate 320 can adjust the optical axis of light of the HID lamp DL in the vertical direction by forwardly and backwardly moving the optical axis adjusting screw 350 inserted in the lamp housing 310 through the screw inserting hole. The inverter 330 is attached to the lower surface of the lamp housing 310 from the lower side so as to close the power line inserting hole 310b and the harness H having one end connected to the inverter 330 is introduced into the lamp housing 310 through the power line inserting hole 310b. 
On the other hand, the high-voltage pulse generator 100 is disposed in the vicinity of the lamp inserting hole 310a of the lamp housing 310 in a state that the HID lamp DL is fitted into the socket B, the other end of the harness H is connected to the IGN connector A, the IGN connector A is directed upward, and the HID lamp DL is received in the lamp housing 310 through the lamp inserting hole 310a. The maintenance cap 340 is attached to the rear wall of the lamp housing 310 in the state that a part of the high-voltage pulse generator 100 is received in the recessed portion 340a. 
[Patent Document 1] Japanese Patent Laid-open No. 2002-217050 (FIGS. 4, 12, and 16)
In recent years, decrease in size of such a vehicle headlight apparatus 300 was advanced, and thus decrease in size of the lamp inserting hole 310a has been required.
However, in the high-voltage pulse generator 100, since the socket B is disposed on the high voltage side of the secondary winding N2 of the pulse transformer PT, a distance between the center of the socket B and a portion farthest from the center in the case 206, that is, a radius of rotation is increased. Thus, when the lamp inserting hole 310a is formed small to correspond to the high-voltage pulse generator 100, the position of the HID lamp DL in the lamp housing 310 is inclined toward an edge of the lamp inserting hole 310a, thereby badly deteriorating workability to fit the HID lamp DL very much. On the contrary, when the position of the HID lamp DL is set in the vicinity of the center of the lamp inserting hole 310a in consideration of the workability to fit the HID lamp DL, the lamp inserting hole 310a should be enlarged in accordance with the radius of rotation of the high-voltage pulse generator 100. Accordingly, it is not possible to accomplish the decrease in size.
Such a problem could be solved by reducing the radius of rotation of the high-voltage pulse generator 100, that is, by disposing the socket B in the vicinity of the center of the case 206, but when the socket B is disposed in the vicinity of the center of the case 206, electronic components 205, . . . must be mounted on the case 206 avoiding and surrounding the socket B in the high-voltage pulse generator 100. As a result, the wiring distance between the electronic components 205, . . . is increased and a path through which pulses pass, that is, a large-current path, is elongated in the pulse generating section denoted by the letter P indicated in FIG. 15, thereby deteriorating the electrical characteristic. Since the high voltage of the secondary winding N2 may leak to the electronic components 205 mounted on the vicinity of the high voltage part of the secondary winding N2 of the pulse transformer PT, an additional insulating process should be performed to the electronic components 205, thereby increasing the manufacturing cost.
That is, in the conventional high-voltage pulse generator 100, when the radius of rotation is decreased in response to the requirement for decrease in size of the lighting apparatus, deterioration in performance due to the deterioration of electrical characteristic or increase in manufacturing cost may be newly caused.